Heat Sink Design Enhancement Improves Power Handling in Surface Mount Components
By Weiping Zheng, Mini-Circuits
High power handing in surface mount components is an important factor in many RF systems where both high power and small device size are required. Mini-Circuits’ high-power surface mount components include models with RF input power maximum ratings up to 100W. However, proper heat sinking of these units is critical to ensure adequate heat transfer to ambient and limit temperature rise. Without proper heat sinking, the unit may overheat and separate from the PCB when operating at high power for long periods of time. Thus, the power handling of the component depends, in part, on the efficiency with which heat can be dissipated to ambient.
This article will present a design enhancement to a Mini-Circuits high-power surface mount coupler in case style AH1647 to improve thermal efficiency and increase the maximum RF input power rating by 10W. The same technique has been applied to other high power surface mount devices to enhance power ratings as well.
Standard Method for Heat Sinking
Mini-Circuits’ SYDC-19-52HP+ is a high power, surface mount bi-directional coupler operating over the 30 to 512 MHz band with 19 dB coupling and 50W power handling. It has an operating temperature range from -40 to +65˚C. The unit is designed with core-and-wire construction and housed in case style AH1647, shown in Figure 1.
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Figure 1: Case Style AH1647
Internally, a ferrite core is mounted on a metal pad, which is securely attached to the PCB of the unit. Heat travels from the core to a metal pad at the base of the unit and should be transferred to ambient to limit rise in temperature. The recommended method to achieve adequate heat transfer is shown in Figure 2 with provisions to be made on the customer’s PCB.
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Figure 2: Method for heat sinking SYDC-19-52HP+ through heat slug in the customer’s PCB.
The customer’s board should be prepared with a rectangular hole measuring 0.24 x 0.245”, matching the dimensions of the metal pad in the unit. The suggested mounting configuration is shown in Figure 3. A metal slug of this size should be fit into the hole in the customer PCB and attached to the bottom of the metal pad at the base of the unit case by soldering. This allows proper transfer of heat from the core of the unit to ambient through the customer PCB. This heat sinking method enables the coupler to operate reliably at up to 50W input power without exceeding the maximum operating temperature of 65˚C (measured on the ground leads).
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Figure 3: Suggested mounting configuration for SYDC-19-52HP+ denoting area of metal pad inside the unit case. Customer PCB should be fitted with a hole of matching dimensions to accommodate a metal heat slug.
Design Enhancement to Improve Power Handling
The SYDC-19-52HP+ was measured using thermal photography to evaluate the thermal properties of the unit. 30, 40 and 50W RF signals were applied continuously to the input for fifteen minutes at the low and high ends of the specified operating frequency range. Temperature measurements were then made at 8 places on the unit to identify the location of the hot spot. Thermal photographs show that the greatest heat rise occurs at the top of the core as successively greater power levels are applied (see Figure 4). Therefore the new design should allow additional heat transfer to ambient from the top of the unit to maintain operating temperature below the rated maximum at higher power.
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To improve heat transfer from the top of the unit, a heat sink was fastened to the top of the ferrite core at the hot spot using a flexible, thermally conductive epoxy adhesive. The modified unit with the new heat sink is shown in Figure 5.
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Figure 5: Heat sink attachment to the top of unit core.
To test the power handling of this design above 50W, the unit was mounted on Mini-Circuits’ test board TB-630+ (see Figure 6) and measured using thermal photography at 50W, 55W, and 63W input power at 4 points across the frequency range. Data from these tests is shown below in Table 1.
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These results show that the DUT can operate reliably with RF input power up to 60W without exceeding the maximum operating temperature rating of 66˚C. While the temperature measured on the epoxy reaches a maximum of 91.9˚C, the temperature range of the epoxy itself is -55 to 230˚C, and there is therefore no risk of thermal failure.
The design enhancement of model SYDC-19-52HP+ illustrated in this article has shown a 10W improvement in the power handling of the device by providing additional heat transfer from the unit core to ambient. Mini-Circuits has created a new standard model, SYDC-19-52VHP+ with electrical performance equal to that of the –HP+ version, but with a maximum RF input power rating of 60W. The same method has also been applied to other high-power surface mount passive devices to increase power handling, including the SYDC-20-171VHP+ 100W coupler and more. The method may be extended to a wide range of high-power surface mount designs utilizing core-and-wire construction to improve thermal efficiency and increase power handling.